The authors performed an experimental study on the retention force in cavities with various depth and width. We used the simple model cavities which were cylindrically formed to eliminate the unexpected influence during experiment, and obtained following results. 1. The retention force in the cavities with parallel wall was vigorous even in the small cavity and increased considerably followed by extending the cavity size. 2. The ratention force resulted more vigorously by that the depth became deeper than that the distance between the lateral walls became longer. 3. Compared with the retention force in the cavities which had the same frictional lateral surface, it was more vigorous in the deeper cavity than in the cavity which was longer distance between the lateral surfaces.